Higher Temperature and Host Age Alter Infection Outcomes in a Multi-Pathogen System

The outcomes of pathogen infection can be sensitive to temperature, interactions with other pathogen species, and host age. Yet few studies have experimentally tested how warming alters infection outcomes for multiple, potentially interacting, pathogen species across host ages. In this study, we conducted a factorial experiment to test how plant age (7-week vs. 13-week plants) and temperature (21°C vs. 29°C) influence infection outcomes of two foliar fungal pathogens with contrasting feeding strategies-Rhizoctonia solani (a necrotroph) and Colletotrichum cereale (a hemibiotroph)-in the grass species tall fescue (Lolium arundinaceum). Contrary to expectations of coinfection with pathogens of opposing life-history strategies leading to increased disease symptoms, coinfection had relatively minor effects across disease metrics. Instead, infection outcomes were driven by host age, pathogen identity and temperature. In plants inoculated with R. solani, higher temperature reduced lesion severity, and independently, severity was less in older plants. C. cereale lesion development showed a strong age × temperature interaction, with older plants being more resistant to disease in cooler conditions but not under warming. In plants co-inoculated with both pathogens, elevated temperature reduced disease severity, and this effect was stronger in older plants. These findings demonstrate that environmental conditions and host age can both interact and outweigh within-host pathogen interactions, highlighting the importance of incorporating host demographic structure in predicting disease responses to climate warming.